This patent application relates to sensors and sensing systems, and more particularly to optical fiber sensors and systems.
Optical fiber cable can be used for sensing temperature, pressure, movement and/or vibration of the fiber. For example, light coupled into a multimode fiber travels along many modes in the optical fiber. In the case of coherent light, there is optical interference between the modes, resulting in a speckle pattern. Disturbances in the fiber result in strain that causes time-varying changes to the optical path lengths among the different modes. Because of the differential path lengths, disturbances of the fiber result in time variation in the speckle pattern. Thus, such a sensor works by monitoring the changes to the speckle pattern, and detecting the instances when the speckle pattern flickers.
Typical applications for optical fiber are found in security for perimeters, pipelines, rails, bridges, and other structures. In one approach, to sense motion or presence of the objects, such as unauthorized persons, a linear sensor including an optical fiber may be slotted into the concrete. However, such a linear sensor is susceptible to false events resulting from vibration caused by nearby aircraft and other heavy equipment.
In another approach, to sense motion or presence of unauthorized persons who try to illegally cross a rail station, for example, optical fibers including a vibration sensor may be attached underneath the pedestrian grating near the rail. This system has proven to be effective at detecting the presence of intruders who try to cross the rail station illegally. However, the system is susceptible to vibration noise caused by approaching and/or departing trains. Although the system can be tuned to reject vibrations caused by underlying noise, e.g., trains, the tuning may be costly, time consuming, and may require a trained technician.
Other situations related to motion detection include sudden infant death syndrome (SIDS). More babies fall victim to sudden infant death syndrome than the combined total of respiratory ailments, heart disease and cancer deaths. Despite extensive research, the exact cause of sudden infant death syndrome is not known. The only effective way of detecting SIDS is to monitor the baby's respiration and, if the baby stops breathing, awaken the baby.
Several monitors exist for alerting parents when a baby has stopped breathing. Some use a pad that detects movement, and others use a device that is attached to the baby's skin. However, these devices are typically not effective, they touch the baby directly and are cumbersome to use.
There is thus a need for improved sensors and sensor systems.